Partial melting
Encyclopedia
Partial melting occurs when only a portion of a solid is melted. For mixed substances, such as a rock containing several different mineral
s or a mineral that displays solid solution
, this melt can be different from the bulk composition of the solid.
Partial melting occurs where the solidus
and liquidus temperatures are different. For single minerals this can happen when they exhibit solid solution, for example in olivine
s between iron
and magnesium
. In rocks made up of several different minerals, some will melt at lower temperatures than others.
Partial melting is an important consideration in geology
with respect to the chemical differentiation
of crustal rocks
. Virtually all rocks on Earth derive from material from the interior of the earth that has been partially melted. The main places where partial melting occurs are subduction
zones, mid-ocean ridge
s and hotspots
. In all these places partial melting is often associated with volcanism
, although some melts do not make it to the surface. Partial melts are thought to play an important role in enriching old parts of the continental lithosphere
in incompatible element
s. Partial melts produced at depth move upwards due to the compaction of the surrounding matrix
.
File:Partial Melting Phase Diagram.svg|Melting of a substance exhibiting solid solution
, with a bulk composition of CB. As the temperature
rises, the solid follows the blue path, and begins melting
at temperature TA. The initial liquid
produced has the composition CL, and its composition follows the red path. At temperature TB the whole solid
has melted.
Image:Partial melting asthenosphere EN.svg|Diagram showing the physical processes inside the Earth that lead to the generation of magma. A to D are different plate tectonic settings.
Image:Coats Fig1.jpg|Subduction in the Aleutian Islands by Robert R. Coats
(1962).
Mineral
A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...
s or a mineral that displays solid solution
Solid solution
A solid solution is a solid-state solution of one or more solutes in a solvent. Such a mixture is considered a solution rather than a compound when the crystal structure of the solvent remains unchanged by addition of the solutes, and when the mixture remains in a single homogeneous phase...
, this melt can be different from the bulk composition of the solid.
Partial melting occurs where the solidus
Solidus (chemistry)
In chemistry, materials science, and physics, the solidus is the locus of temperatures below which a given substance is completely solid...
and liquidus temperatures are different. For single minerals this can happen when they exhibit solid solution, for example in olivine
Olivine
The mineral olivine is a magnesium iron silicate with the formula 2SiO4. It is a common mineral in the Earth's subsurface but weathers quickly on the surface....
s between iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
and magnesium
Magnesium
Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...
. In rocks made up of several different minerals, some will melt at lower temperatures than others.
Partial melting is an important consideration in geology
Geology
Geology is the science comprising the study of solid Earth, the rocks of which it is composed, and the processes by which it evolves. Geology gives insight into the history of the Earth, as it provides the primary evidence for plate tectonics, the evolutionary history of life, and past climates...
with respect to the chemical differentiation
Igneous differentiation
In geology, igneous differentiation is an umbrella term for the various processes by which magmas undergo bulk chemical change during the partial melting process, cooling, emplacement or eruption.-Primary melts:...
of crustal rocks
Crust (geology)
In geology, the crust is the outermost solid shell of a rocky planet or natural satellite, which is chemically distinct from the underlying mantle...
. Virtually all rocks on Earth derive from material from the interior of the earth that has been partially melted. The main places where partial melting occurs are subduction
Subduction
In geology, subduction is the process that takes place at convergent boundaries by which one tectonic plate moves under another tectonic plate, sinking into the Earth's mantle, as the plates converge. These 3D regions of mantle downwellings are known as "Subduction Zones"...
zones, mid-ocean ridge
Mid-ocean ridge
A mid-ocean ridge is a general term for an underwater mountain system that consists of various mountain ranges , typically having a valley known as a rift running along its spine, formed by plate tectonics. This type of oceanic ridge is characteristic of what is known as an oceanic spreading...
s and hotspots
Hotspot (geology)
The places known as hotspots or hot spots in geology are volcanic regions thought to be fed by underlying mantle that is anomalously hot compared with the mantle elsewhere. They may be on, near to, or far from tectonic plate boundaries. There are two hypotheses to explain them...
. In all these places partial melting is often associated with volcanism
Volcanism
Volcanism is the phenomenon connected with volcanoes and volcanic activity. It includes all phenomena resulting from and causing magma within the crust or mantle of a planet to rise through the crust and form volcanic rocks on the surface....
, although some melts do not make it to the surface. Partial melts are thought to play an important role in enriching old parts of the continental lithosphere
Lithosphere
The lithosphere is the rigid outermost shell of a rocky planet. On Earth, it comprises the crust and the portion of the upper mantle that behaves elastically on time scales of thousands of years or greater.- Earth's lithosphere :...
in incompatible element
Incompatible element
An incompatible element is a term used in petrology and geochemistry to describe an element that is unsuitable in size and/or charge to the cation sites of the minerals, and is defined by the partition coefficient between rock-forming minerals and melt being much smaller than 1..During the...
s. Partial melts produced at depth move upwards due to the compaction of the surrounding matrix
Matrix (geology)
The matrix or groundmass of rock is the finer grained mass of material in which larger grains, crystals or clasts are embedded.The matrix of an igneous rock consists of finer grained, often microscopic, crystals in which larger crystals are embedded. This porphyritic texture is indicative of...
.
Gallery
File:Partial Melting Phase Diagram.svg|Melting of a substance exhibiting solid solution
Solid solution
A solid solution is a solid-state solution of one or more solutes in a solvent. Such a mixture is considered a solution rather than a compound when the crystal structure of the solvent remains unchanged by addition of the solutes, and when the mixture remains in a single homogeneous phase...
, with a bulk composition of CB. As the temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
rises, the solid follows the blue path, and begins melting
Melting
Melting, or fusion, is a physical process that results in the phase change of a substance from a solid to a liquid. The internal energy of a substance is increased, typically by the application of heat or pressure, resulting in a rise of its temperature to the melting point, at which the rigid...
at temperature TA. The initial liquid
Liquid
Liquid is one of the three classical states of matter . Like a gas, a liquid is able to flow and take the shape of a container. Some liquids resist compression, while others can be compressed. Unlike a gas, a liquid does not disperse to fill every space of a container, and maintains a fairly...
produced has the composition CL, and its composition follows the red path. At temperature TB the whole solid
Solid
Solid is one of the three classical states of matter . It is characterized by structural rigidity and resistance to changes of shape or volume. Unlike a liquid, a solid object does not flow to take on the shape of its container, nor does it expand to fill the entire volume available to it like a...
has melted.
Image:Partial melting asthenosphere EN.svg|Diagram showing the physical processes inside the Earth that lead to the generation of magma. A to D are different plate tectonic settings.
Image:Coats Fig1.jpg|Subduction in the Aleutian Islands by Robert R. Coats
Robert R. Coats
Robert Roy Coats was born in Toronto, Canada, and grew up in Marshalltown, Iowa and Seattle, Washington. He graduated valedictorian of his high school class in Seattle at the age of 16, and attended the University of Washington, where he received both a B.S. and M.S. degree in Geology and Mining...
(1962).